This invention is related to an electrical discharge machine (EDM) apparatus and method for providing a reverse taper bore in a stationary workpiece by wobbling an electrode holder while an electrode acts upon the workpiece.
EDM is a well-known and widely used machining technique. It operates through electrical discharges from an electrode. The process takes place in the presence of a liquid dielectric fluid such as de-ionized water. An electrode in the shape of an elongated rod, thin wire, or shaped article is put into close contact with the workpiece. Through an electrical potential difference, arcing occurs between the workpiece and the electrode which causes erosion of the workpiece material in a desired manner. EDM processes are used in numerous machining applications and are especially desirable for applications involving high accuracy, extremely hard workpieces, low applied loading of the workpiece, or deep bores in a workpiece.
Some machining projects pose difficulties for existing EDM technology. One example relates to fuel injection nozzles of the type used in certain internal combustion engines, such as diesel engines. In a typical diesel fuel injection tip, multiple fuel injection holes are bored near the end and around the circumference of the fuel injection tip to atomize highly pressurized fuel as it is introduced into a combustion chamber. The bores are conventionally formed as cylindrical bores of generally constant diameter. It is thought, however, that bores having other shapes may be better suited to some applications. In particular, it is believed that a reverse-taper of the fuel injection tip holes, where the interior orifice of the fuel injection bores (inside the fuel injector tip) have a larger diameter than the orifice diameter at the exterior of the injection tip, would provide superior results. Such reverse taper designs appear to increase fuel economy, increase the life of the injectors, and address problems relating to fuel cavitations and emissions.
Due to the size and configuration of many workpieces, including fuel injection tips, fuel injection borings must be made from the outside of the workpiece. A certain amount of deviation from a constant bore diameter can be provided with a wire EDM by controlling spark voltage and other parameters. Due to edge effects, however, this technique tends to produce a barrel-shaped internal bore. A need exists for an EDM machine capable of producing precise reverse-taper or cone-shaped bores in articles such as fuel injection tips.